Differential amplifiers are electronic devices for amplifying a voltage difference between two input signals. A differential amplifier typically includes two electrical paths that are independently coupled to a voltage source at one end, and that are together coupled to a voltage or current source at an opposite end. Each electrical path usually includes a transistor element and a resistance element. Two differential input signals may be applied to the transistor elements, and a differential output signal may be extracted from each electrical path between its transistor element and resistance element. The two electrical paths are usually designed to substantially match each other, such that components common to both differential input signals are cancelled and a voltage difference signal between the input signals is amplified.
A conventional differential amplifier may have its pair of emitters (in case of bipolar transistors), or its pair of sources (in case of metal-oxide-semiconductor (MOS) transistors) tied together to a current source. Such current source supplies a fixed tail-current to each transistor of the differential amplifier. When supplied with a fixed tail-current, the differential amplifier may have a gain of only 6 to 12 dB. In order to increase the gain of the differential amplifier, differential cascaded amplifiers are necessary, thus making the circuit more complex.
In addition, for wideband frequency circuit applications, a conventional high amplifier requires frequency compensation components, which may be achieved with the addition of compensation capacitors in the circuit. This further complicates the operation of the differential amplifier and deteriorates other dynamic performance characteristics of the amplifier, such as its response time.
As will be explained, one embodiment of the present invention provides a differential amplifier having a high gain of approximately 50 to 60 dB. This high gain is advantageously achieved with a single amplification stage, without any need for cascaded arrangement of amplifier stages.